Capacitive touch screens include a conductive layer on a substrate. A capacitive touch screen typically consists of a glass substrate with a thin-film conductive coating on top of the glass. On top of this is an anti-scratch glass overcoat. This seals in the sensor electronics and makes the device resistant to scratches.
Circuits at the edges of a typical capacitive screen uniformly distribute a low-voltage AC field over the conductive surface. When a finger or other conductive object disrupts that field, the controller measures the change in current flowing from each corner and calculates the X and Y coordinates, which indicates the point of the screen where the user is touching.
Typically, the conductive layer is a substantially transparent conductor such as indium tin oxide (ITO), antinomy tin oxide (ATO), or zinc oxide (ZnO), which are examples of substances generally referred to as transparent conductive oxides (TCOs). However there are some problems with TCOs in capacitive touch panels. The typical sheet resistance of TCOs in certain applications is listed in Table A. Capacitive touch systems require relatively high sheet resistance compared to other applications.
TABLE A.Application for TCOResistivity (Ohms per square)Electronic displays such LCDs 10-100Resistive touch systems100-500Capacitive touch systems1000-2500
In order to increase the sheet resistance of a TCO, a thinner film of the TCO is typically deposited. For example, an ITO film with a sheet resistance of 20 ohms per square is about 500 nm thick. An ITO having sheet resistance of 350-400 ohms per square is only about 35-30 nm thick. Uniformity, durability, and physical continuity of a deposited TCO are generally degraded as the film is coated thinner. For example, ITO is generally not coated at a resistivity of 1000-2000 ohms per square because it has to be so thin that it may not be durable, uniform or physically continuous. Performance is further degraded if the substrate top surface is non-uniform, which is often the case. ATO can be coated thicker for higher sheet resistance. For example, a coating of ATO that is 25-30 nm thick results in a sheet resistance of about 2000 ohms per square. However, ATO has inferior optical properties compared to ITO, such as lower transmission.
As a result, an improved conducting layer of a capacitive touch panel is needed that provides durability and good optical properties.